June 19, 2012
01:00 PM (EDT)

News Release Number: STScI-2012-27

Most Quasars Live on Snacks, Not Large Meals

June 19, 2012: Black holes in the early universe needed a few snacks rather than one giant meal to fuel
their quasars and help them grow, a new study shows. Quasars are the brilliant beacons of
light that are powered by black holes feasting on captured material, and in the process,
heating some of the matter to millions of degrees. The brightest quasars reside in galaxies
distorted by collisions with other galaxies. These encounters send lots of gas and dust
into the gravitational whirlpool of hungry black holes. Now, however, astronomers are
uncovering an underlying population of fainter quasars that thrive in normal-looking spiral
galaxies. They are triggered by black holes snacking on such tasty treats as a batch of gas
or the occasional small satellite galaxy.

A census of 30 quasar host galaxies conducted with two of NASA's premier observatories,
the Hubble Space Telescope and Spitzer Space Telescope, has found that 26 of the host
galaxies bear no tell-tale signs of collisions with neighbors, such as distorted shapes.
Only one galaxy in the sample shows evidence of an interaction with another galaxy. The
galaxies existed roughly 8 billion to 12 billion years ago, during a peak epoch of black-hole growth. The study bolsters evidence that the growth of most massive black holes in the early universe was fueled by small, long-term events rather than dramatic short-term major mergers.